Modulation of porosity in a solid material enabled by bulk photoisomerization of an overcrowded alkene

Abstract

Incorporation of photoswitchable molecules into solid state materials hold promise for fabrication of the responsive materials, the properties of which can be controlled ondemand. However, the possible applications of these materials are limited, due to the restrictions imposed by the solid state environment on the incorporated photoswitches, which render the photoisomerization inefficient. Here we present responsive porous switchable framework materials based on a bistable chiroptical overcrowded alkene incorporated in the backbone of the rigid aromatic framework. Due to the high intrinsic porosity, the resulting framework readily responds to a light stimulus as demonstrated by solid-state Raman and reflectance spectroscopies. Solid state 13 C NMR spectroscopy highlights efficient and quantitative bulk photoisomerization of the incorporated light-responsive overcrowded olefins in the solid material. Taking advantage of the quantitative photoisomerization, the porosity of the framework, and as a consequence gas adsorption, can be reversibly modulated in response to light and heat. Inspired by biological systems, a vast number of artificial molecular machines and switches, capable of elaborate structural dynamics, have been developed. 1-4 However, in solution stimulated molecular motion is inevitably overwhelmed by isotropic thermal noise which thereby precludes any form of collective action and the extraction of macroscopic work, since the molecules behave independently from one another. 5-7 On the contrary, solid state organization can translate stimuli-controlled nanoscopic changes into useful material properties and preludes to practical outputs. Thus, a challenging endeavour of current research is to organize molecular machines and switches in the solid state to impede random, thermal motion and amplify mechanical effects along multiple length scales: this effort requires reliable strategies, allowing for restricting random motion and limiting the degrees of freedom to selected modes, without impairment of the rotary or switching functions. 8-10 One strategy to achieve these goals is the use of solid porous materials 11-15 which can incorporate switchable moieties and provide the free volume essential for unhindered dynamics, thereby serving as a static scaffold for the flexible components. Indeed, it was demonstrated recently that molecular rotors 16-23 , shuttles 24 , switches 25 and motors 26 can display their designed motion while incorporated

All-Photochemical Rotation of Molecular Motors with a Phosphorus Stereoelement

“…Overcrowded-alkene-based molecular motors, which operate through a unidirectional rotary cycle consisting of alternating photochemical and thermal steps ( Figure 1 A), 2 are an appealing class of actuators for such machines because of their ability to convert light energy into mechanical force at the nanoscale. The design and study of a wide range of motors utilizing this operating principle have enabled the engineering of functional systems including dynamic metal–organic and covalent frameworks, 3 hierarchical supramolecular assemblies with artificial muscle-like function, 4 and liquid crystals with rotating helicity capable of mechanically rotating microscale objects. 5 …”

Photoresponsive porous materials

“…More recently, similar changes in the gas uptake were observed upon reversible [2+2] photocycloaddition in poly(aryl vinylene) 2D COFs. 125,126…”

Anionic Polymerization in Porous Organic Frameworks: A Strategy to Fabricate Anchored Polymers and Copolymers

“…In recent years,t he achievement of materials endowed with high surface areas and porosity on the nanoscale,h as provided challenging adsorption capacity,g as separation, catalytic activity,l uminescent properties,a nd stimuli-responsiveness. [1][2][3][4][5][6][7][8][9][10][11][12] Theu nprecedented surface area and porecapacity of new-generation porous materials (metal-organic frameworks (MOFs), porous organic polymers (POPs), and porous molecular crystals (PMCs)) allows intimate interaction with guest molecules such as monomers,p roviding the opportunity to grow in situ polymer chains under confinement, yielding polymeric nanostructured materials with an exquisite control over the growth and architecture of polymer chains. [13][14][15] Notable results span from stereochemical and constitutional control to shape retention and morphological transcription.…”